Frost, weazel, and qts have it.

I finally ripped the motherboard out of the case and jumpered the power on... and voila, the fans started spinning, and the system POSTed successfully. Clearly it's the case causing a short with the motherboard. No amount of finessing could get this motherboard to work with this case (an InWin mid-tower), so the case got the heave ho. Another, older case seems to work so far... I was able to mount and power up the motherboard in it. I haven't attempted anything beyond that, but it appears that this problem, at least, is licked.
Thanks for the help, everyone!

-- Todd/Leviathn